Well, "slithy" means "lithe and slimy." "Lithe" is the same as "active." You see it's like a portmanteau - there are two meanings packed up into one word.
Lewis CarrollDo you know, I always thought unicorns were fabulous monsters, too? I never saw one alive before!" Well, now that we have seen each other," said the unicorn, "if you'll believe in me, I'll believe in you.
Lewis CarrollDo you hear the snow against the windowpanes, Kitty? How nice and soft it sounds! Just as if some one was kissing the window all over outside. I wonder if the snow loves the trees and fields, that it kisses them so gently? And then it covers them up snug, you know, with a white quilt; and perhaps it says, 'Go to sleep, darlings, till the summer comes again.' And when they wake up in the summer, Kitty, they dress themselves all in green, and dance about - whenever the wind blows.
Lewis Carroll